formatted lerna completion and used helper function

completion/available/lerna.completion.bash

@@ -1,5 +1,8 @@
 #!/bin/bash
 # Lerna autocompletion.
+
+if _command_exists lerna
+then
     function _lerna_completions() {
         local cur compls
 
@@ -17,3 +20,4 @@ function _lerna_completions() {
         COMPREPLY=($(compgen -W "$compls" -- "$cur"))
     }
     complete -o default -F _lerna_completions lerna
+fi